Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, example, coelenterates. An undifferentiated layer, mesoglea is present in between the ectoderm and the endoderm (Figure: a).

Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals (platyhelminthes to chordates Figure: b).